Reduction of fog formation in an electrophotographic light sensitive sheet



United States Patent "ice 3,540,885 REDUCTION OF FOG FORMATION IN ANELEC- TROPHOTOGRAPHIC LIGHT SENSITIVE SHEET Satoru Honjo, Yasuo Tamai,and Seiji Matsumoto,

Saitama, Japan, assignors to Fuji Shashin Film Kabushiki Kaisha,Kanagawa, Japan No Drawing. Filed Jan. 27, 1967, Ser. No. 612,076 Claimspriority, applicationggapan, Jan. 27, 1966,

Int. c1. Gll3g 5/00 US. Cl. 96-1 6 Claims ABSTRACT OF THE DISCLOSUREBefore charging the surface of an electrophotographic light sensitivesheet for forming electrostatic latent images thereon by light exposurefollowed by liquid development, the layer is coated with an insulatingliquid such as kerosene to reduce formation of fogs.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a new method of electrophotography which utilizes a liquiddeveloper.

In the xerographic process an electrostatic latent image is formed on aphotoconductive insulating layer and the image is then converted to avisible image by development. Here the polarity of the latent image isprimarily determined by the constitution of the layer. For example, anelectrostatic image of negative polarity is preferably formed on a layerwhich contains an n-type photoconductor, such as ZnO and CdS. Such alayer exhibits better electrophotographic properties i.e., a higher darkresistivity and a higher photoconductivity, when a negative surfacecharge is applied on the surface, giving a visible image of bettercontrast.

On the other hand, an amorphous selenium coating exhibits bettercharacteristics when charged to positive polarity. This selectivity isattribued to a higher mobility of holes compared with the mobility ofelectrons in the layer.

Having only one polarity for one type of electrophotographic materialcauses the following disadvantage. When one wants to get positivereproductions from any arbitrary original images, positive or negative,utilizing a determined type of electrophotographic recording member fedinto a copying machine, one must prepare and charge the machine with twokinds of developer. The exchange of the developers is troublesome, andinevitably is accompanied by an undesirable contamination of them.

Thus it would be highly desirable to provide an electrophotographicrecording member which could be charged to any desired polarity andstill show satisfactory electrophotographic properties. In that way onecould always obtain positive reproductions, regardless of originalsused, by changing the polarity of the corona charging and utilizing onekind of developer. Moreover, if it is desired to get a negative from apositive, one may again change the polarity of the charging.

SUMMARY OF THE INVENTION This invention provides a method by which anelectrophotographic recording member can be charged in both polarities,even though the material is otherwise preferably charged to only onedetermined polarity. This method is characterized by utilizing a liquiddeveloper.

This method comprises covering a photoconductive insulating layerprovided on a conductive support, such as an aluminum plate or slightlyconductive support, such as paper with an insulating liquid which isinactive to 3,540,885 Patented Nov. 17, 1970 the layer, then chargingthe liquid bearing layer electrostatically, exposing the layer to alight image thereby forming an electrostatic latent image, anddeveloping the layer in or by applying a liquid developer whichcomprises charged dispersed powders and an insulating liquid mediummiscible with the above liquid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrophotographicrecording member used in the present invention comprises a conductive orslightly conductive support and a photoconductive insulating layerprovided thereon; here the support must be conductive at least at thesurface on which the photoconductive layer is formed. Accordingly, aninsulating film or plate having a conductive surface, such as alaminated metal layer, may also be used.

Photoconductive insulating layers are prepared by coating a mixturecomprising an insulating binding material and a pulverizedphotoconductor, or by depositing a volatile photoconductor in vacuum. Agreat number of photoconductors appear in the published literature, anyof which may be used for the present method. There may be provided aninsulating overcoating which can transmit at least part of an activeradiation to the photoconductive layer.

The insulating organic liquid used in the present method has to beinactive to the photoconductive layer, not dissolving or stronglyswelling the layer, and must have a dielectric constant less than about5. It may be volatile or non-volatile at room temperature; however, toohigh an evaporation rate makes the succeeding development processdifiicult. In the liquid, insulating resinous materials, waxy compounds,or other coloring matters may be dissolved or dispersed. Theintroduction of these materials depends on the particular purposes.Though such a liquid comes into contact with and difiuses into thecarrier liquid of the liquid developer which is successively applied onthe recording layer, it still remains in high concentration at thesurface of the recording layer so that it can also be used to controlthe development conditions. Among suitable liquids used for covering aphotoconductive layer are aliphatic hydrocarcarbons, polymethylsiloxanes, etc.; a small amount of polar or weakly polar liquids, suchas ketones and esters can be used. Resinous materials or plasticizersmay also be permitted, In general, those which are suitable as carrierliquids for the liquid developer may be used for this purpose.Accordingly those which have dielectric constants less than 5, andspecific resistivities higher than 10 to 10 ohms-cm. are desirable. Atoo thick liquid layer on the photoconductive layer sometimes invitesbackground densities and lowering of contrast. A very thin layer ispreferred in practice, i.e., the preferable thickness lies between 0.5and 15 microns. In the case of a commercially available ZnOelectrophotographic paper, it may be moistened with a suitableinsulating liquid at its surface, then contacted with a sheet ofblotting paper or cloth so that almost all of the liquid is removed.When the surface of a recording member is porous, some of the appliedliquid is impregnated into the layer; however, in such a case goodresults can still be obtained.

The eliect of such a liquid layer is clearly demonstrated in thefollowing testing procedure. A sheet of an electrophotographic paperutilizing photoconductive ZnO is partly moistened with an insulatingliquid, while the remaining area is kept untreated. This sheet issubjected to a positive corona charging in darkness, exposed to a lightimage, and developed with a liquid developer. The treated area gives animage of sufficient density and contrast free of background which can becompared with one obtained by negative charging, while the untreatedarea gives a faint image in a rather dense background.

Liquid developers used in this method must contain toner particlescharged to the desired polarity, and the carrier liquids must containsuch components which are miscible with the liquid coated on therecording member.

The present method can also be used when the electrophotographic memberis charged to the polarity at which it exhibits betterelectrophotographic behavior. In this case, there is not observed such aremarkable improvement as in the case of the reverse polarity, but stillthe presence of an insulating liquid layer sometimes increases thecontrast in the resulting visible image. Another improvement is observedwhen the electrophotographic member is in a light adapted state. Theinsulating liquid layer may be present on the photoconductive coatingwhen the pre-exposure is carried out, or it may be introduced betweenthe pre-exposure step and the charging step at darkness. Anyway, it isonly necessary that the insulating liquid layer be present on thephotoconductive layer when the layer is subjected to corona charging.Theexistence of the liquid layer proved to give far better contrast inthe resulting visible image. But on a strongly fatigued coating, aliquid layer is sometimes ineffective.

The mechanism of the present method is presently difficult to explaintheoretically. A probe of an electrometer closely placed on a ZnOcoating covered by a thin layer of kerosene, which gave a clear imagewith positive surface charging, detected a surface potential of a verylow value similar to an uncovered coating.

Utilizing the present method one can always obtain positivereproductions from positive and negative originals with one kind ofdeveloper by only changing polarities of charging. A liquid applyingroller may be installed before the charging unit in the copying machine:the roller may be one like a moistening roller in an offset duplicatingmachine. In the case of ZnO paper, since the liquid application isnecessary for positive charging, the roller may be moved to contact thesurface of the ZnO paper only when the polarity of charging is positive.In some cases, the roller may always be kept in contact with the surfaceof an electrophotographic member.

The presence of an insulating liquid layer proved to decrease thefrequency of spot generation in photoconductive coating due to localbreakdown during charging.

EXAMPLE 1 A liquid developer was prepared by dispersing in 1000 cc. ofkerosene an intimate mixture comprising 1.5 g. of carbon black (AsahiCarbon Co.), 2 g. of polyvinyl acetate, and 10 cc. of methyl ethylketone. The dispersed particles in this developer acquired positivecharges and deposit on a negatively charged latent image. On the otherhand, an electrophotographic member was prepared by coating a paper basewith a homogeneous mixture comprising the following ingredients:

Parts by weight Photoconductive ZnO 100 Epoxyester of fatty acids fromdehydrated caster oil (oil length 40%, epoxy resin usedEpikote Cobaltnaphthenate 0.1

The member was dried after coating at 50 C. for 4 hours. Before chargingit was kept in the dark for more than 12. hours to be dark-adapted.About half of the sheet was covered with kerosene (the thickness of thekerosene film was about 5 microns) the other half was left untreated.This sheet was subjected to negative corona charging of 7000 v., thenexposed to a light image, and then developed with the liquid developerdescribed above. A visible image was obtained on the whole area of thesheet, but a lower background density was observed at the treated area.

4 E AM L The same sheet as described in the Example 1, half treated withkerosene, was subjected to a positive corona charging of 8000 v., andprocessed in a manner similar to Example 1. A visible image appeared atthe area which had been covered with kerosene, while at the remainingarea a very faint image was observed.

EXAMPLE 3 A similar processing as in Example 2 was repeated except thefollowing liquid developer was used in the present example.

Cyanine green (Toyo Ink C0.)0.5 g. Kerosene200 cc.

Cyclohexane-65 cc.

In this developer dispersed cyanine green was charged negatively and wasattracted to a positively charged latent image. A green positivereproduction of a positive original was obtained only at the coveredarea in the sheet.

EXAMPLE 4 A similar processing as in Example 1 was repeated, but in thisexample a pre-exposure of 100 lux from a tungsten lamp for 3 seconds wasgiven to the half-covered sheet before charging. The development usingthe liquid developer described in Example 1 combined with negativecharging resulted in a legible image only at the covered area.

EXAMPLE 5 EXAMPLE 6 A similar processing as in Example 2 was repeatedexcept the covering liquid was changed to a mixture of 1 part ofkerosene and 1 part of soybean oil. A reversal development occurred. Asimilar result was obtained by using peanut oil in place of soybean oil.

EXAMPLE 7 The electrophotographic paper described in Example 1 washomogeneously covered by a layer of kerosene 18 microns in thickness.Half of the sheet was pressed to a blotting paper and a major part ofthe kerosene was removed. Then it was subjected to positive coronacharging with 8000 v., potential. A subsequent image exposure anddevelopment with the liquid developer described in Example 1 gave avisible image on the entire area of the sheet. But a lower backgrounddensity was observed at the area where the blotting paper was pressed.From weight measurement, the thin kerosene coating proved to have about0.7 micron thickness.

EXAMPLE 8 A photoconductive plate was prepared by depositing selenium invacuo on an aluminum plate of 20 cm. x 30 cm. size. The thickness ofselenium was about microns. This plate was covered with a layer ofkerosene 4 microns thick, and then was subjected to negative coronacharging with 7000 v. potential. A subsequent image exposure anddevelopment with the liquid developer described in Example 1 gave aclear visible image, which was then transferred electrostatically to areceiving sheet.

5 EXAMPLE 9 A similar procedure was repeated as in Example 8 except thatthe developer used was changed to that described in Example 3. Areversal development occurred giving a clear image.

What we claim is:

1. An electrophotographic method which comprises applying to aphotoconductive insulating layer on a conductive or slightly conductivesupport an insulating liquid which does not dissolve said layer, havinga dielectric constant less than S, and a specific resistivity higherthan 10 cm., in a thickness of from about 0.5 micron to about 15microns, electrostatically charging the liquidbearing photoconductivelayer only after said insulating liquid is applied, exposing the layerto a light image to provide an electrostatic latent image in the layer,and developing the thus exposed layer with a liquid developer havingfine toner particles dispersed in an insulating liquid medium misciblewith the aforesaid insulating liquid.

2. The electrophotographic method as claimed in claim 1 wherein saidinsulating liquid is kerosene.

3. The electrophotographic method as claimed in claim 1 wherein saidinsulating liquid is a mixture of kerosene and soybean oil.

4. The electrophotographic method as claimed in claim 1 wherein said.insulating liquid is a mixture of kerosene and peanut oil.

5. The electrophotographic method of claim 1 further comprising the stepof removing part of said insulating liquid from said layer prior tocharging.

6. The electrophotographic method of claim 5 wherein said partialremoval of said insulating liquid is accomplished by blotting saidinsulating liquid with an absorbent material.

References Cited UNITED STATES PATENTS 3,227,076 l/1966 Castle 101-l49.43,357,828 12/1967 Moe 961 3,368,894 2/1968 Matkan et a1. 96-1 3,412,24211/1968 Giaimo 250-49.5

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant ExaminerUS. Cl. X.R.

